Various types of surface illuminant units have been proposed and put into practical use as surface illuminants for illuminating transmission type liquid crystal displays, etc. from their rear. Surface illuminant units are classified in two principal types, edge light type and direct type. The surface illuminant units of these two types are different in the mode of conversion of a non-surface illuminant into a surface illuminant.
For example, in a surface illuminant unit of direct type, cold cathode ray tubes arranged in parallel are used to illuminate a transmission type display element, such as an LCD panel, from its rear. The cold cathode ray tubes and the transmission type display element such as an LCD panel are properly spaced, and a diffuser and a combination of two or more optical sheets for converging light are placed in this space.
Such a conventional transmission type display is poor in light-converging properties although the number of optical sheets required is great. In order to compensate for the light-converging properties, LCD panels have been improved so that even obliquely entering light produces high-quality images on the LCD panels.
However, this way of improving LCD panels is disadvantageous in that it causes reduction in optical efficiency and that it makes the structure of LCD panels complex, which leads to increase in cost.
In particular, the light intensity on (the brightness of) an LCD panel tends to become non-uniform (non-uniformity of brightness) depending on the distance from cold cathode ray tubes (whether a certain point on an LCD panel is close to a cold cathode ray tube or to the space between cold cathode rays tubes arranged in parallel). In order to prevent such non-uniformity of brightness, if the distance between the cold cathode ray tubes and the LCD panel is made greater, the display inevitably has a greater thickness, and if the degree to which light is diffused is increased, or the LCD panel is made to transmit a limited amount of light, optical efficiency decreases.
Japanese Patent Laid-Open Publications No. 119703/1993 and No. 242219/1999, for example, are technical literatures on the prior art. In the surface illuminant units described in these publications, a light-shielding member (a lighting curtain, a light-shielding dot layer) is employed in order to maintain the uniformity of the light intensity (brightness). However, this technique impairs optical efficiency as described above.
Further, a method using a sheet having lenticular lenses on both sides is described in Japanese Patent Laid-Open Publication No. 347613/1994, for example. Such a sheet is for controlling the diffusion of light in two directions, and does not have the function of converging light. Therefore, an optical axis on the surface of an LCD panel changes according to its position relative to cold cathode ray tubes, and the brightness of a display screen varies (non-uniformity of brightness) depending on the position (direction) from which the screen is viewed.
Furthermore, Japanese Patent Laid-Open Publication No. 318003/1988 discloses a film having an array of prisms for converging light. When this film having prisms is used, not only a brightness peak appears at an exit angle of around 0 degree, but also a second brightness peak appears at a great exit angle of about 60 to 80 degrees. The use of such a film has been disadvantageous in that a large amount of light leaks when a screen displays black color, resulting in lowering of contrast. This film having prisms has also been disadvantageous in that it causes abrupt changes in both vertical and horizontal viewing angle characteristics and that the brightness of a display screen changes greatly.
In addition, a conventional surface illuminant unit has light emission characteristics that peak when light exits in the direction of the normal to the plane of emergence of the surface illuminant. Therefore, the use of such a conventional surface illuminant unit for a television or the like, which is usually viewed from above or the front, has been disadvantageous in that optimum viewing angle characteristics cannot be obtained, and that optical efficiency is poor.